Change in crystallization mechanism at the glass transition of colloidal spheres

Abstract

THE crystallization of 'hard' spheres, which interact only through repulsive forces on contact, is a purely entropic effect and provides a model system for testing theories of freezing transitions in liquids generally¹. Micrometre-sized colloidal particles can be made to approximate hard spheres by grafting of polymers to their surfaces¹. Their slow dynamics make them attractive systems for the study of crystallization kinetics^2–4. As well as undergoing crystallization^1,2, such particles also exhibit a glass transition at a well-defined volume fraction φ_g, which we have shown previously to be related to the cessation of large-scale diffusion on experimental timescales^5,6.. Here we show that the glass transition coincides with a change in the mechanism of crystallization. At volume fractions less than φ_g isometric crystals are nucleated homogeneously throughout the sample, whereas above φ_g crystals forming slowly in the metastable glass phase are highly asymmetric. Regular rocking of the suspension above the glass transition induces more rapid formation of plate-like crystals, indicating that their nucleation is shear-induced.